The Core of this application consists of two major components. In the first component, support is requested for continued operation of the transgenic mouse facility which has been an integral part of this program since its inception. This facility provides for embryo microinjections and the surgery required for generating transgenic mice;, breeds and produces pseudopregnant recipient mice for implantation of injected murine embryos; and provides help in the protocols for injections of pregnant mare's serum and chorionic gonadotrophin for superovulation. During the past five years, this facility, has been extensively used by Drs. McDevitt. Davis, Weissman and Parnes and during the coming five years will be used by these investigators as well as by Drs. Jones and Goodnow in a series of transgenic mouse lines described in the Core proposal. The second component of the Core facility is the fluorescence activated cell sorter, which provides support to the laboratories of all the investigators in this program. This is a state of the art facility run by Drs. Herzenberg and Parks. It includes two multiparameter FACS sorters. a four parameter FACSscan flow cytometer and extensive computing facilities required for the analysis of data coming from analytical runs on the sorter. This is a unique resource which has provided excellent technical support to all of the laboratories in the program as well as to a number of other laboratories at Stanford Medical School. This facility has used been used very heavily by the investigators on this grant and all of the proposed projects require continued high usage for the experiments described.